This invention relates to methods of cracking hydrocarbon feedstocks in the presence of a cracking catalyst. More particularly, the invention relates to the fluid catalytic cracking of plural hydrocarbon feedstocks having diverse cracking characteristics.
A number of processes for the cracking of hydrocarbon feedstocks via contact at appropriate temperatures and pressures with fluidized catalytic particles are known in the art. These processes are known generically as "fluid catalytic cracking" (FCC).
Relatively, lighter molecular weight and lower boiling point hydrocarbons, such as gas oils, are typically preferred feedstocks for FCC operations. Such hydrocarbons generally contain fewer contaminants and have a lower tendency to produce coke during the cracking operation than heavier hydrocarbons. However, the relatively low content of such light hydrocarbons in many current crude mixes has lead to the attractiveness of heavier hydrocarbons, for example residual oils, as feedstocks to the FCC operation. One problem with the heavier hydrocarbons, however, is that these materials generally contain a higher level of metals which tend to contaminate the catalyst and increase the yield of coke during the cracking operation. In addition, the heavier hydrocarbons also tend to contain a greater abundance of coke precursors such as asphaltenes and polynuclear aromatics which result in increased coke lay.
Several attempts have been made to minimize the negative impact that heavy hydrocarbon feedstocks tend to have on FCC operation. For example, U.S. Pat. No. 4,552,645-Gartside et al eliminates the problem by avoiding the FCC unit altogether, instead routing the heavy hydrocarbon to a stripper/coker wherein such material is thermally cracked at high temperatures. U.S. Pat. No. 4,422,925-Williams et al is directed to an FCC process having a plurality of hydrocarbon feedstocks introduced at diverse locations in a riser type reactor in the presence of a zeolite catalyst. The lowest molecular weight feedstock is introduced in the bottom of the reactor. Hydrocarbon feedstocks having the highest tendency to form coke are introduced i the uppermost section of the riser and are exposed to the lowest reaction temperature and the lowest catalyst to oil ratios.
U.S. Pat. No. 4,218,306-Gross et al is assigned to the assignee of the present invention and is incorporated herein by reference. The disclosure of Gross is consistent with the Williams teaching insofar as it requires converting relatively low coke producing gas oils in a lower initial portion of a riser and then a higher coke producing feedstock, such a recycle oil, is introduced in an upper section of the riser.
In typical FCC configurations the feedstocks to be cracked were introduced either all together at the bottom of the riser or with the heavier fractions being introduced into the upper portions thereof. In direct contrast to the state of the art as presented by the above described patents, applicants have discovered that it is beneficial and desirable that the heavier, higher molecular weight hydrocarbon feedstocks, i.e., those feedstocks generally having a relatively high tendency to produce coke, be introduced into the riser at a location which is relatively upstream of the location at which the lighter, lower molecular weight feedstocks are introduced.
The methods of the present invention may also be used to optimize the slate of reaction products resulting from a single individual feedstock, independently of whether that feedstock is cracked alone or jointly with other individual feedstocks. For example, in certain refinery operating modes only a single unblended hydrocarbon stream may be available as FCC feedstock. According to one embodiment of the present invention, such a feedstock is first separated into light and heavy fractions. The separate fractions are then introduced into the reactor such that the heavy fraction enters the riser at a point relatively upstream of the light fraction. In this way, the conditions under which the light and heavy fractions are cracked may be optimally adjusted according to the teachings of the present invention.
According to certain preferred embodiments of the present invention, a relatively heavy hydrocarbon feedstock, such as residual oil, is used as a fresh feed to an FCC cracking unit for initially contacting suspended, hot and relatively active regenerated catalyst at an elevated temperature in a disperse phase catalytic conversion zone. Thereafter, a lighter hydrocarbon feedstock, such as gas oil, is injected into a downstream portion of the disperse phase suspension. Thus the relatively heavy hydrocarbon feedstock will be in contact with the catalyst for only a portion of the residence time available in the riser before coming in contact with a lighter hydrocarbon feedstock. Although it is contemplated that the heavy hydrocarbon may be introduced at any location within the riser provided its relative position to the lighter feedstock is maintained, according to certain preferred embodiments the relatively heavy hydrocarbon is introduced into the bottom of the riser where it is contacted with catalyst. The catalyst introduced into the bottom of the riser generally comprises freshly regenerated catalyst which enters the riser at an elevated temperature relative to the hydrocarbon feedstocks. The catalyst temperature entering the riser is generally greater than about 1100.degree. F., preferably between about 1200.degree. and 1450.degree. F., while the temperature of the hydrocarbon feedstock is considerably less, generally less than about 800.degree. F., preferably between about 300.degree. and 600.degree. F. Applicant has found that segregation of the feedstocks as taught by the present invention produces heavy hydrocarbon reaction temperatures which may be readily optimized according to the particular feedstock mix to be cracked. As the term is used herein, heavy hydrocarbon reaction temperature refers to the mix temperature in the heavy hydrocarbon reaction zone of the riser. As the term is used herein, heavy hydrocarbon reaction zone refers to the portion of the riser between the heavy hydrocarbon injection location and the light hydrocarbon injection location. As will be appreciated by those skilled in the art, the intimate contact between the heavy hydrocarbon and the hot catalyst which occurs at the entrance to the heavy hydrocarbon reaction zone will result in an initial heavy hydrocarbon/catalyst mix temperature which is between the temperature of the heavy hydrocarbon and the hot catalyst, depending upon the catalyst to oil ratio. For the purpose of convenience, the initial mix temperature in the heavy hydrocarbon reaction zone is herein defined as the initial adiabatic temperature of the mixture. This temperature is readily calculated by performing an enthalpy balance around the entrance to the heavy hydrocarbon reaction zone and by assuming no heat of reaction at the entrance. As is also understood by those skilled in the art, the temperature in the heavy hydrocarbon reaction zone generally decreases as the suspension passes upwardly through the zone and the endothermic reaction proceeds. Thus the temperature profile of the hydrocarbon/catalyst mix generally decreases continuously along the length of the heavy hydrocarbon reaction zone. The extent of the temperature decrease is a function of many parameters, including feedstock and catalyst characteristics and reaction zone configuration. The effect of these parameters and hence the mix temperature at the exit of the heavy hydrocarbon reaction zone can generally be estimated by those skilled in the art for any particular set of conditions. At the interface of the heavy hydrocarbon reaction zone and the light hydrocarbon reaction zone there will be a relatively discontinuous temperature drop due to the quenching effect of the light hydrocarbon feedstock. For the purpose of convenience, the initial mix temperature in the light hydrocarbon reaction zone is herein defined as the initial adiabatic temperature at the light hydrocarbon injection location. This temperature is readily calculated by performing an enthalpy balance around the entrance to the light hydrocarbon reaction zone and by assuming no heat of reaction at the entrance.
The methods of the present invention thus allow the heavier hydrocarbons to be initially cracked at temperatures which are higher than would otherwise be possible in a typical FCC process. Since only a portion, preferably a minor portion, of the total hydrocarbon charged to the riser is initially contacted with the hot, freshly regenerated catalyst, the temperature of the initial catalyst/hydrocarbon suspension is higher than the temperature which would result if both the heavy hydrocarbon and light hydrocarbon feedstocks were introduced together at a single location in the riser. Accordingly, one important aspect of the present invention resides in "blasting" the heavy hydrocarbon feedstock to catalyst mix temperatures which are higher than otherwise attainable without simultaneously subjecting the light feedstock or fractions to such unusually high temperatures. High temperature cracking of relatively heavy hydrocarbon feedstock increases the production of the preferred products at the expense of undesirable coke, without exposing the light hydrocarbons to such temperatures. Initial mix temperature in the heavy hydrocarbon reaction zone are preferably from about 1050.degree. to about 1250.degree. F., and more preferably from about 1100.degree. F. to about 1200.degree. F.
According to a further step of the present invention, a lighter hydrocarbon feedstock is introduced into the riser at a location which is downstream with respect to the heavy hydrocarbon feed injection location. The injection point for the light hydrocarbon feed is preferably selected to ensure that the contact time in the heavy hydrocarbon reaction zone or the blast zone of the riser is short relative to the contact time available in the entire riser. In this way, introduction of the lighter hydrocarbon feed into the suspension acts as a quench for the heavy hydrocarbon reaction and prevents overcracking which would otherwise occur at the relatively high temperatures existing in the heavy hydrocarbon reaction zone. Although applicant does not intend to be bound by or to any particular theory, applicant believes that processes according to the present invention result in vaporization and primary cracking of the asphaltenes, polynuclear aromatics, and other high molecular weight components of the heavy hydrocarbon feedstock at relatively high temperatures which promote the formation of desirable products at the expense of coke. Moreover, the period of contact at such relatively elevated temperatures, i.e., the contact time in the heavy hydrocarbon reaction zone, is made relatively short by the downstream introduction of the lighter hydrocarbon feedstocks which tend to quench the reaction and thereby reduce the reaction temperatures. In this way, undesirable secondary cracking of the reaction products produced in the heavy hydrocarbon reaction zone is minimized.
Accordingly, one important aspect of the present invention resides in reducing the temperature of the hydrocarbon/catalyst suspension at the exit of the heavy hydrocarbon reaction zone. Thus, injection of light hydrocarbon feedstock into the riser produces an initial light hydrocarbon reaction zoned temperature which is relatively low compared to the reaction temperature at the exit of the heavy hydrocarbon reaction zone. As the term is used herein , light hydrocarbon reaction zone temperature refers to the temperature in the light hydrocarbon reaction zone of the riser. For the purposes of convenience, the portion of the riser reactor down stream of the introduction of the light hydrocarbon feedstock is referred to as the "light hydrocarbon reaction zone", although this term is in no way limiting with respect to the type of hydrocarbon feedstocks which may be additionally introduced into the riser downstream of the light hydrocarbon feedstock injection location. At the interface of the heavy hydrocarbon reaction zone and the light hydrocarbon reaction zone there will be a relatively discontinuous temperature drop due to the quenching effect of the light hydrocarbon feedstock. For the purpose of convenience, the initial mix temperature in the light hydrocarbon reaction zone in herein defined as the initial adiabatic temperature at the light hydrocarbon injecting location. This temperature is readily calculated by performing a enthalpy balance around the entrance to the light hydrocarbon reaction zone and by assuming no heat of reaction at the entrance. Once again, the present invention is not limited to any particular temperature range in the light hydrocarbon reaction zone since this temperature will also be effected by many conditions, including but not limited to feedstock properties, desired FCC product rate and catalyst circulation rate. Nevertheless, applicant has found that the initial mix temperature in the light hydrocarbon reaction zone is preferably from about 950.degree. to about 1050.degree. F., and more preferably from about 980.degree. to about 1020.degree. F. In terms of quenching capacity, applicant has found that the introduction of the light hydrocarbon into the suspension is preferably sufficient to assure a reduction in suspension temperature of at least about 50.degree. F., and more preferably at least about 100.degree. F., with even better results achieved with even more quenching, e.g., there are benefits to operating with 150.degree. to 250.degree. F. of quench.
According to a further step required by some embodiments of the present invention, the hydrocarbon/catalyst suspension, after the introduction of the light hydrocarbon feedstock, is further passed through the riser reactor for a contact time which is relatively long compared to the contact time in the heavy hydrocarbon reaction zone. According to certain preferred embodiments, the contact time in the heavy hydrocarbon reaction zone is preferably less than about half the contact time in the light hydrocarbon reaction zone, and more preferably less than about one-third the contact time in the light hydrocarbon reaction zone. According to certain embodiments, the contact time in the heavy hydrocarbon reaction zone is preferably less than about one-fifth the contact time in the light hydrocarbon reaction zone.